There are many instances in which it is desirable to monitor a sensitive area of a building, installation or other indoor or outdoor space for the presence of unauthorized intruders. Some known arrangements for this purpose will now be discussed.
GE Interlogix Sensors and Detectors, of 12345 SW Leveton Dr., Tualatin, Oreg. 97062, U.S.A. manufactures a break-glass sensor known as the ‘5150 Shock Sensor’ designed to raise an alarm when an intruder attempts to gain entry to a sensitive area by breaking a window on its perimeter. Any attempt to break the window bends a piezoelectric element in the sensor, and the electric current generated by the element in response is detected by a monitoring circuit which triggers an alarm circuit. However, this device only monitors certain points around the perimeter of the restricted space (the windows). If an intruder gains access to the space by, for example, following an authorized person through a security door without being challenged, this device can provide no indication that a security breach has occurred.
Alarm Lock, of 345 Bayview Avenue, Amityville, N.Y. 11701, U.S.A., manufactures a door alarm device known as the ‘SirenLock PG21’. This device is a lock containing a magnetic reed switch, which is affixed to the door to be monitored, coupled with a magnet that is attached to the frame of the door to be monitored. When the door is opened, the switch is moved away from the magnet, and the switch opens—if the person opening the door has not disabled the lock using a key, this action activates an alarm circuit to indicate an unauthorized entry. Again, this device only monitors certain areas around the perimeter of the restricted space (the doors), and cannot bring the presence of unauthorized people within the space to the attention of security personnel via an alarm.
Texecom Limited, of Texecom House, 559 Wilbraham Road, Manchester M21 0AE, Great Britain, manufactures a passive infra-red detection device known as the ‘Reflex’. The device comprises an infra-red sensitive element placed behind a lens with a certain field-of-view. A monitoring circuit samples the electrical properties of the element, which change when infra-red light of the correct wavelength (that of the infra-red light emitted by the human body) falls on to it. When a person walks into the field-of-view of the device, the infra-red energy emitted by their body is picked up by the sensor, and the monitoring circuit detects the resulting change in the sensor's electrical properties, triggering an alarm. Because the sensor reacts to all people entering the sensor's field-of-view, it can only be used during periods when no authorized personnel will be present in the area covered. Therefore, these sensors are typically used to monitor areas within a sensitive space outside of normal operating or working hours.
Kalatel Inc., of 4575 Research Way, STE 250, Corvallis, Oreg. 97333, U.S.A. manufactures a video surveillance system known as the ‘Digiplex’ system that allows operators to view images captured by a set of cameras placed around a sensitive space. An operator may be able to use the system to spot an intruder in a camera image amongst the authorized users of that space, but for the system to be effective during periods when authorized users are likely to be present in the sensitive area (for example, during the day) the operators must be able to identify all authorized users on sight. Furthermore, the use of human operators (rather than an automated monitor) will result in relatively high running costs for the system.
Whilst these security sensors can monitor either the perimeter of a sensitive space at all times, or regions within that space at certain times when no authorized personnel are present, they are unable to effectively monitor regions within a space populated by authorized personnel for the presence of intruders.
The invention seeks to overcome at least some of the disadvantages of the prior art.